It is well known and accepted medical practice to treat patients afflicted with respiratory disorders by use of mechanical ventilatory therapy. Treatment of patients with acute lung injury and/or respiratory failure often is complicated by any number of physiologic derangements imposed by the disease process or the physical injury, and morbidity and mortality can be quite high for such patients. A consensus has developed in the medical community that mechanical ventilatory techniques produce detrimental side effects, and that the observed morbidity and mortality often are secondary to the complications produced by the conventional modes of mechanical ventilatory therapy.
Patients who require total mechanical ventilatory support rarely will tolerate continuous positive airway pressure (CPAP) levels in excess of 20 CM H.sub.2 O because conventional assisted respiration, in combination with CPAP therapy, results in continual application of elevated mean and peak airway pressures at which lung capacity exceeds normal functional residual capacity (FRC). This sort of mechanical ventilation therapy also may lead to detrimental cardiovascular effects such as negation of the natural augmentation of venous return that attends spontaneous respiration, and in decreased cardiovascular output.
Such patients also commonly exhibit markedly decreased lung compliance which results in extremely high airway pressures during mechanical inspiration thereby greatly increasing the risk of barotrauma (for example, subcutaneous emphysema, pneumothorax and pneumopericardium). Renal, hepatic, and cerebral function may also be impaired in patients who are ventilated with high peak and mean airway pressures.
Patients with acute respiratory failure also commonly exhibit, in addition to decreased lung volume and decreased lung compliance, mismatching of ventilation and perfusion causing arterial hypoxemia, tachypnea and increased work of breathing. This clinical presentation leads to the impression that these patients require external ventilatory assistance, usually with positive pressure mechanical ventilation.
Among the conventional mechanical ventilation techniques are assist mechanisms, intermittent mandatory ventilation (IMV), positive end-expiratory pressure (PEEP), and high frequency low-tidal volume therapy such as applied in infant ventilation. These have been proposed to improve ventilatory therapy and to thereby decrease the rate and severity of complications. Yet in spite of these attempts, the mortality rate from respiratory failure has changed little in many years.
Among the prior art patents known to applicant pertaining to ventilatory techniques and apparatus are the following: U.S. Pat. Nos. 4,552,141; 4,502,502; 4,466,433; 4,351,329; 4,333,452; 4,316,458; 4,182,366; 4,180,066; 3,800,793 and 3,358,680.